Wafer throughput and particle counts are key metrics for any semiconductor manufacturer's yield enhancement program. Recent advancements in diffuser technology have helped manufacturers enhance these metrics while improving the attributes for most vacuum processes. These processes include dry etch, chemical vapor deposition (CVD), physical vapor deposition (PVD), rapid thermal processing (RTP) and Epitaxial deposition (Epi).
The use of membrane diffuser technology for the vent-up of vacuum chambers has dramatically decreased the required vent time compared to a chamber soft vent without a diffuser. An early implementation of this technology was used on 200 mm batch-style loadlocks that had an inherently large internal volume. The loadlock was prone to long vent cycles to prevent particle contamination.
As the industry transitioned to a 300 mm wafer platform, factories increased their development of single-wafer loadlocks (SWLL) in an effort to boost tool throughput. Gas diffusers with ultra fine filtration membranes solved these issues. Compared to the 200 mm batch-style loadlocks, the SWLLs had extremely low internal volumes and were designed to cycle vacuum to atmosphere very quickly. With the low volumes inherent in the SWLL, the velocity of the incoming vent gas became critical, since any particles on the bottom of the loadlock chamber would easily sweep onto the wafer should they be hit with a high velocity gas. Particles are typically present in the loadlock due to mechanical wafer handling devices and environmental exposure. Gas diffusers allowed a large, uniform volumetric flowrate of gas into the loadlock chamber at low downstream gas velocities.
Loadlocks or other chambers may use membrane diffusers, a screen, frit and/or soft vent procedure to control the flow into the chamber to reduce particle counts while maintaining throughput levels at a low cost with minimal downtime. However, while these membrane diffusers, screen or frit can distribute gas in the chamber, they are relatively open and provide little resistance to gas flow into the chamber.
One method used to control particle disturbance in semiconductor vacuum process tool platforms is a two-step venting process, which implements a “soft” vent followed by a standard vent. The soft vent is typically conducted using a second line equipped with a flow restrictor to minimize the flowrate and bleed gas into the chamber until a certain pressure is reached inside the loadlock. This helps reduce the disturbance of particles. Once a set pressure is reached in the loadlock, a second valve is actuated to complete the venting process and bring the pressure of the loadlock to atmosphere. Depending on the volume of the loadlock chamber, the soft vent stage alone can take anywhere from a few seconds to several minutes to complete.
This method can be used where chamber throughput is low. However where tool owners are required to increase wafer throughput due to capacity constraints or to enhance Overall Equipment Effectiveness (OEE) this two-step vent process requires too much time. While many of the critical variables that influence wafer throughput are fixed, (such as the process times, robot speed and loadlock pump down speed) the time to vent up of a chamber or loadlock may become the rate-limiting step to wafer throughput. This is especially true with shorter process times or if dual batch loadlocks are not working in parallel. One approach is to provide a rapid pressure increase by boosting the flowrate of gas. However, with a standard screen or open porous material like a frit, the gas velocity at the chamber entrance will be high and non-uniform, resulting in the disturbance of unwanted particles that have settled in the chamber.
The situation also occurs where vent up time is not a throughput limiting step. In this case, the tool owner is faced with more stringent particle requirements or observes a spike in particles on the wafers in the loadlock. The focus then becomes yield enhancement and the goal is to reduce the particle adders on the wafers. Common approaches to the particle problem on installed system loadlocks have included complete loader rebuilds, performing additional series of wet cleans, upstream filter replacements and screen diffuser replacements, which often do not yield the desired goal.
Membrane diffuser technology allows a rapid but controlled vent up of loadlocks, cool down, transfer and process chambers from vacuum to atmospheric pressure while protecting the wafer integrity. For example, this technology has reduced vent times on a variety of 200 mm vacuum process tool platforms by an average of 65 percent and significantly reduced particle adders—all by maximizing the volumetric flow and minimizing the velocity of ultra pure gas.
Diffuser products are designed using fine porous media, which uniformly spreads the gas flow across a large area, resulting in lower velocities at the chamber entrance. The porous media also serves as a particle filter, removing particles down to 0.003 μm from the incoming gas at high volumetric flows. The result is ultra-clean, diffused gas delivered to the process chamber, which minimizes on-wafer defects.
A membrane diffuser allows the duration of the soft vent to be significantly reduced or even eliminated, and increases the volumetric flowrate into the loadlock to dramatically reduce the overall vent cycle. It should be noted that the diffuser does not affect the pump down cycle. With a diffuser, the decrease in vent time does not come at the expense of higher gas velocities and particle disturbance as typically seen with screens or coarse porous frits. The membrane media for diffusers previously used was designed to uniformly spread gas flows across a large area relative to a standard gas line, a series of drilled holes, or coarse screens. With these membranes high volumetric flows can be achieved with low uniform gas velocities, however the flow could be turbulent rather than laminar. Laminar flow provides less risk of particle re-distribution in the chamber onto sensitive substrates.
A comparison of different components used to create uniform flow were made from measurements that were taken using an ultrasonic probe in the fluid path exiting the component. The results show how the membrane diffuser is more effective compared to a frit or screen under the same volumetric flow conditions. The result of lower gas velocities is the decrease in particle counts (or adders) to the wafer during a loadlock or chamber vent. Particle results taken on wafers prior to and after installation of the diffuser technology on a chamber or loadlock. The combination of an ultra pure filter and fine membrane gas diffuser allowed this dramatic reduction in particle occurrence.
One of the most difficult questions to answer in chamber and diffuser design is what gas velocities are acceptable during a vent to atmospheric pressure with respect to particle re-entrainment. This is a problem compounded by the various mechanisms that adhere a particle to the surface and the varying sizes and shapes of these particles. This can make it extremely difficult to determine the flow required to lift a particle. In addition, since the fluid flow conditions are dynamic, the boundary layer conditions also are active and contribute additional uncertainty in the fluid force available to lift a particle. Further, to determine optimum venting is a relatively difficult analytical problem to fully solve. Physical geometries are fairly complicated, making Computational Fluid Dynamics (CFD) modeling difficult. Additionally, fluid flow may be present in various flow regimes including molecular flow, viscous flow (both laminar and turbulent) and even as shock wave fronts. Lastly, the size or adhesiveness of settled particles on the floor or walls of a loadlock or chamber make it difficult to determine the exact target for nearby fluid velocities to minimize re-entrainment.
Simplified CFD models can provide a general picture of the fluid flow in a loadlock or chamber leading to the best compromise between short vent time, minimal fluid velocity, physical placement of a diffuser, shape of the diffuser, chamber or loadlock geometry and vent-up parameters (e.g. soft vent use). The controlled permeability of the diffuser membrane can also be used to make the fluid flow uniform across the membrane and offers proper resistance to flow in this configuration. The design engineer controls diffuser location during install, membrane shape and membrane permeability.
Mykrolis Microelectronics Applications Note AN1501ENUS recognizes that vent times can be provided that range from less than 0.2 seconds per liter of chamber volume to 4 seconds per liter depending upon the diffuser and supply pressure. Supply pressures of about 40 psig were reported and maximum inlet pressures with outlet to vacuum of 100 psig or less were reported. It also recognizes that for ultra sensitive applications where the overriding concern is preventing the disturbance and redistribution of particles existing in the chamber that a low face velocity is desired and suggested that large area diffusers or multiple diffusers be used. This takes up chamber space and can be expensive. High pressure differential can damage some of these porous membranes. A membrane and operating conditions to achieve laminar flow during vent up of a chamber from vacuum were not disclosed.
U.S. Pat. No. 5,908,662 discloses a processing system including a vacuum chamber and at least one tube disposed through a wall of the vacuum chamber. A gas diffuser is disposed in said tube, possibly at the end of the tube and/or outside the chamber. The gas diffuser is formed from a porous, possibly metallic, material which includes a plurality of microscopic holes whereby gas entering or leaving the vacuum chamber through the tube has a reduced force compared to if the gas diffuser was not present. A membrane and operating conditions to achieve laminar flow during vent up of a chamber from vacuum were not disclosed.
U.S. Patent Application Pub. No 2004/0083588 discloses a vacuum/purge operation of a loadlock chamber prevents an eddy phenomenon from occurring in the chamber and thereby prevents wafers from being polluted and damaged by particles in the chamber. A vacuum pump for providing the loadlock chamber with vacuum pressure, and a gas supply for providing the chamber with purge gas are connected to the loadlock chamber by an exhaust line and a gas supply line, respectively. At least one control valve is installed in each of the lines. At the time the state of pressure in the loadlock chamber is to be changed, the loadlock chamber is provided with both the vacuum pressure and the purge gas at rates that are inter-dependent to establish a flow of gases towards and into the exhaust line. Then, the supplying of one of the vacuum pressure and the purge gas is gradually reduced and cut off. This method of venting a chamber requires large amounts of gas and control feedback between the vacuum pump and gas source to achieve vacuum/purge operation which is expensive. Large differential pressures may exist across the chamber during vent-up, but a membrane diffuser that minimizes variation in gas velocity is not used or disclosed.
U.S. Pat. No. 7,112,237 is directed to porous composite materials comprised of a porous base material and a powdered nanoparticle material. The porous base material has the powdered nanoparticle material penetrating a portion of the porous base material; the powdered nanoparticle material within the porous base material may be sintered or interbonded by interfusion to form a porous sintered nanoparticle material within the pores and or on the surfaces of the porous base material. Preferably this porous composite material comprises nanometer sized pores throughout the sintered nanoparticle material. The disclosure is also directed to methods of making such composite materials and using them for high surface area catalysts, sensors, in packed bed contaminant removal devices, and as contamination removal membranes for fluids. This membrane utilizes bonding of the fine layer to the housing which can be stressed by the differential pressures across the membrane during a chamber vent.
Others have reported that rapid changes in pressure on the introduction of purge gas into a vacuum or removal of gas from a chamber can result in turbulent flow. This can result in the deposition of particles from the chamber onto substrate surfaces. Reduction in flow turbulence during venting diminishes the chance that chamber particles will be disturbed and deposited onto substrates in the chamber. A membrane and operating conditions to achieve laminar flow and minimal variation in mass flow during vent up of a chamber from vacuum were not disclosed.
Diffuser membranes currently in load lock chambers or other vacuum chambers have been used to reduce gas velocity compared to an orifice plate, open porous frit, or a tube having one or more macroscopic holes. However, these membranes can result in a large variation in gas velocity and/or mass flow through the diffuser membrane during vent-up of the vacuum chamber depending upon the pressure of the gas upstream of the membrane. Current diffuser membranes or screens are designed to be relatively open (large pores or openings) to provide low resistance to gas flow while in some instances providing the particle retention of typical gas filters. The relatively open structure of these diffuser membranes shortens vent-up times and increases processing throughput, it also lowers stress and possible cracking of these membranes which can be caused by repeated vent cycles and the pressure differential across the membrane between the vacuum chamber and vent gas source. These more open membranes, frits, and screens while reducing membrane stress and shortening vent-up time, can lead to large variations in gas mass flow over the vent cycle. When gas velocity or mass flow varies sharply, flow conditions can fluctuate from laminar to turbulent or laminar to transitional flow and the sharp variation in flow can cause the re-distribution of chamber particles onto sensitive substrates in the chamber.